The present invention relates generally to the field of marine seismic surveying. More particularly, in one or more embodiments, this invention relates to electromagnetic linear actuators used in acoustic vibratory sources for marine seismic surveying.
Seismic sources, including vibratory sources, are utilized in geophysical exploration on land and in water covered areas of the Earth. Signals generated by these sources travel downwardly into the Earth and are reflected from reflecting interfaces in the subsurface. Reflected signals are then detected by signal detectors, typically hydrophones or geophones, on or near the Earth's surface.
Most of the seismic sources employed today in marine surveying are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The frequency content of those sources is controllable only to a small degree, and different sources are selected for the generation of different frequency ranges for different surveying needs. These impulsive sources have a limited acoustic output in the very low frequency band; especially below 5 Hz the output is very poor. Another type of seismic source used in marine surveying includes acoustic vibratory sources, including hydraulically powered sources and sources employing piezoelectric or magnetorestrictive material. While acoustic vibratory sources may have better frequency control than impulsive sources, they are also limited at low frequency content. In general, all sources applied in seismic surveying are typically limited at low frequency content, in that no or very limited energy is generated below 5 Hz.
It is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sounds waves are attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. There has been a long standing need in seismic surveying for powerful, low frequency acoustic vibratory sources operating in the frequency band up to 5 Hz.
Accordingly, there is a continuing need for improved acoustic sources for use in marine surveying.